The present invention relates to a positive / negative image forming method and apparatus used for a type of electrophotographic image forming device, for example, a copying machine.
Conventionally, an electrophotographic image forming device, for example, a copying machine, has adopted either a positive image forming method or a negative image forming method. In the former method, a positive image is formed, i.e. the image of white parts of the original is formed as white while the image of black parts of the original is formed as black. In the latter method, a negative image is formed, i.e. the image of black parts of the original is formed as white while the image of white parts of the original is formed as black.
In the positive image forming device, for example, a main charger and a developing bias are negatively polarized with the voltage of the developing bias being smaller than that of the main charger, while the toner is positively polarized. The positively polarized toner is attracted to the charged portion of an electrostatic latent image, that is, the portion where the voltage is lower than that of the negatively polarized developing bias. Thus, a positive image corresponding to the original is formed. The positive image is transferred to a copy paper via a transference charger being with the transference charger being oppositely (in this case negatively) polarized to that of the toner. On the other hand, in the negative image forming device, for example, a main charger, toner, and a developing bias are negatively polarized so that the negatively polarized toner is attracted to the non-charged portion of the electrostatic latent image on the surface of a photoreceptor drum. Thus, a negative image corresponding to the original is formed. The negative image is transferred to a copy paper via a transference charger with the transference charger positively charged being.
The above conventional devices have disadvantages in that both of the devices have specifically single-purpose mechanisms for forming either a positive image or a negative image. So, the whole device must be changed to the other type according to the image required or both a positive image forming device and a negative image forming device should be furnished, which results in high-cost.